Vehicle engineers are constantly looking to make improvements to steering systems. These systems are especially critical in commercial and industrial trucking since these trucks haul massive loads over a variety of terrain. The trucks may be required to cover long distances at relatively high speeds on the highway or make short runs in tight spaces on the jobsite. The steering system affects the ability for the truck to corner, as well as the ability to keep the truck moving in a straight line rolling down the highway. A properly designed steering system can also significantly reduce tire wear by avoiding skidding or “tire scrub.”
Some work trucks, an example being shown in FIG. 1, are designed with dual or twin front axles 5, 10, each carrying steerable front wheels. A forward steer axle 5 is shown at the front end of the vehicle under the cab 3 and a rearward steer axle 10 is rearward of the forward steer axle, forward and rearward being relative to the vehicle. These twin front axles increase the load capacity of the truck and improve the weight distribution of the loaded truck between the front axles and the rear axles, allowing the truck to more easily traverse roadways with loose surfaces.
To achieve the least possible tire wear, the proper angle of deflection for each wheel needs to be established based on the desired radius of the turn. A schematic of these deflection angles is shown in FIG. 2. The optimal angle of deflection will require each wheel to follow its own circular path around the same center. As a result, the inner wheels, relative to the curve, should be turned more than the outer wheels, and the wheels of the rearward front axle should be turned more than the wheels of the forward front axle.
One prior art steering system for these trucks with dual front axle steering is shown in FIG. 3. The steering system includes a steering gear 20, a pitman arm 24, and a steering intermediate arm 28. The pitman arm 24 anchors two linking rods 32, 33; one linking rod attaches to the intermediate arm 28 and the other attaches to a forward steering arm 36 of the forward front axle 5. Extending rearward from the intermediate arm 28 is another linking rod 34 that connects to a rearward steering arm 37 of the rearward front axle 10. Also extending rearward of the intermediate arm 28, is a power assist piston 40 that uses hydraulics to assist the pivoting motion of the intermediate arm 28.
As can be seen from FIG. 3, the intermediate arm 28 of the prior art fills a large packaging space due to its method of attachment and orientation with respect to the frame. The intermediate arm 28 pivots relative to the vehicle frame 44 about a horizontal axis 29. This motion requires a long arm to achieve the desired degree of travel for the points of attachment to the linking rods 32, 33, 34. Further, several substantial bends in the rod 32 are required to connect the linking rod 32 between the pitman arm 24 and the intermediate arm 28. The bends significantly reduce the stiffness of the linking rod 32, which adversely affects the responsiveness of the steering system.
Accordingly, there is a need for a steering system that links the movement of a first and second set of truck wheels while providing the proper cornering geometry. There is a need for a system having sufficient stiffness for improved straight run performance. There is also a need for a system which can be packed into the already crowded space around the front end of the truck chassis.